1. Field of the Invention
This invention relates to a small and thin step-up transformer suitable for use in an inverter for lighting a cold-cathode tube illuminating the back side of a liquid crystal display or the like.
2. Description of the Related Art
FIG. 12 shows the construction of a conventional step-up transformer for inverters. A primary, low-voltage-side winding L1 and a secondary, high-voltage-side winding L2 are wound on a hollow bobbin 2 having a plurality of flanges 1 in such a way as to be separated from each other by the flanges 1. The secondary winding L2, in which high voltage is generated, is wound in a plurality of stages due to the flanges 1 so as to reduce the difference in electrical potential between the adjacent wire sections, thereby preventing dielectric breakdown. Inserted into bore 3 of the bobbin 2 are central feet 6 and 7 of two E-shaped cores 4 and 5, which abut each other and are secured in position in this condition. The bobbin 2 and the cores 4 and 5 are fixed to an insulating base (not shown) having terminals embedded therein.
A transformer of this type must be formed thin so that it can be arranged in a narrow space in a liquid crystal display device. As a result, the cross section of the bore 3 of the bobbin 2 needs to be a flat, rectangular configuration, so that the winding length is getting longer as compared with bore 3 being not flat like a square or a circle, resulting in an increase in conductor resistance and deterioration in efficiency. In such a conventional rectangular transformer, 5 mm is the limit in the reduction in thickness because of the base attached from below to the bobbin and cores. A further reduction in thickness and a further increase in width will result in an excessive increase in copper loss. Further, since the secondary winding L2 has to be wound in a plurality of stages, the winding operation is rather complicated and the volume of the entire transformer is increased.